There are a number of radio services in which it is necessary or desirable that a radio transmitting and receiving antenna be capable of operation at any frequency within a relatively broad band of frequencies.
The demand for more efficient use of radio frequency spectrum space can be satisfied by time division multiplexing, and that can be made more efficient if frequency multiplexing is added. The limited ability to change transmitter (and receiver) frequency is no longer the boundary condition that limits effective use of the latter. The advent of the microprocessor controlled frequency synthesizers and broadband power amplifiers has removed the transmitter as the limiting factor to frequency multiplexing, and the major problem has become antenna band width limita- tion.
Much work has been done to provide broadband antennae with limited success. Certainly, commercial success has been limited. Less work appears to have been done and, certainly, even less success has been achieved in broadbanding directional antennae that employ passive radiators or parasitic elements.
While the advance of frequency multiplexing points up one need for broadband antenna systems, most radio communication services still employ or are based on single frequency carriers whether or not the carrier is transmitted. Those radio services also are in need of better broadband antenna systems. That is true, among others, of the military and amateur radio services. In one example, the 75-80 meter amateur band extends from 3.5 MHz to 4.0 MHz, a range of essentially thirteen percent of midband frequency. Amateur licensees may operate at any frequency within that band, but not all of them can do that. An antenna presents a different impedance to a transmitter and receiver at different frequencies, and the impedance of the standard dipole and Marconi type antennae changes more from 3.5 MHz to 4.0 MHz than most modern transmitters can accommodate. The standard, or reference, antenna is one-half electrical wave length long. It is divided at the center, and the feed lines are connected one to each leg. Mounted one-quarter wave length, or multiple thereof, above a perfectly conductive ground, such an antenna presents a 73 ohm radiation resistance at the feed point. If the frequency is increased, the impedance seen by the feed line is a combination of resistance and inductive reactance. If the frequency is decreased, the impedance presented to the feed line is a combination of resistive and compacitive reactance.
The transmitter output circuit is a resonant circuit or untuned network set to the transmission frequency. When the antenna presents a reactive load to the transmitter output circuit, the effective output circuit is untuned with attendant high SWR generated on the feed line. The result can be generation and radiation of harmonic signals, excessive and damaging voltages and circulating currents, and reduction of efficiency and radiated energy. The transmitter can be protected by the inclusion of "matching" networks in the transmission feed line from the transmitter to the antenna, but the use of a matching network solves only part of the problem. In practice, it is necessary to retune the transmitter or the matching network when changing transmitting frequency more than two or three percent.
The amount of retuning that is required when changing frequency across a radio service band can be reduced by the use of a "broadband" antenna. Such an antenna incorporates variations from the standard dipole or Marconi antenna the effect of which is to minimize the increase in reactive impedance at the antenna feed point with frequency at frequencies higher and lower than the resonant frequency. An antenna exhibits characteristics similar to those exhibited by a lumped resonant circuit and, just as in the lumped resonant circuit, the change in antenna impedance with frequency is minimized if the Q of the system is low. The Q is reduced if the antenna conductors are increased in diameter relative to antenna length. It is also known that the band width, the frequency deviation that can be accommodated without undue increase in reactance, increases as radiation resistance increases.
Unfortunately, practical circumstances result in a decrease rather than an increase in radiation resistance. The reference dipole exhibits exactly 73 ohms radiation resistance only in free space high above a perfect ground. In practice, radiation resistance is decreased by an increase in radiator diameter relative to length, by reflective objects mounted near the antenna, and by a decrease in antenna height below one-quarter wave length, particularly below 5 MHz. It is common practice, when radiation resistance cannot be calculated or otherwise accurately predicted, to assume a radiation resistance of 50 ohms. That assumption having been made, it is convenient to feed the antenna with a 50 ohm coaxial, non-radiating transmission line. It is customary to interconnect the antenna and the transmission line with a transformer called a "balun" arranged to provide a transistion from the balanced antenna to the unbalanced transmission line.
In addition to lowering antenna Q, attempts have been made to increase antenna band width by the incorporation of elements in the antenna system, the function of which is to introduce reactive impedance with frequency change which is the conjugate of the impedance change exhibited by the antenna with that frequency change. To accomplish that result, some attempts have been made to incorporate low Q resonance circuits in the antenna legs.
A variation of that approach is used in the "double bazooka" antenna which was developed as a radar antenna during World War II. The double bazooka is formed by a length of coaxial cable the outer braid of which is severed and separated at the midpoint along its length. The braid at each side of the separation is connected to a respectively associated side of the feed line. At the ends of the coaxial cable, the center conductor and the outer shield braid are connected together. The result is like a folded dipole antenna except that the center conductor does not radiate. The antenna is tuned by the addition of lengths of wire to each end of the coaxial section so that the combination of braid and end section is electrically one-quarter wave length long in air. Each half of the coaxial cable is also one-quarter wave length long electrically, but, because the propogation velocity is less in the coaxial cable, it is physically shorter by the length of the end section. In the true double bazooka, the end sections are formed of open wire transmission line.
The band width limitations of the dipole antenna extend to the Marconi antenna. The Marconi antenna, in most common forms, is an odd number of electrical quarter waves long and is mounted vertically. The earth reflects radio waves in a manner similar to reflection of light by a mirror. In the case of the vertical Marconi antenna, an "image" of the antenna can be considered to exist below ground. The combination of the antenna and its image resemble a dipole except that the contribution of the image portion to radiation resistance measured at the center point of the dipole (base of the Marconi portion) depends upon the conductivity of earth. The analogy to mirror reflection is exact. Just as the mirror can accomplish reflection of three dimensional objects, although it occupies only a plane, so the image antenna is reflected from a plane in the earth. That action can be simulated by electrical conductors extending radially in a plane from the base of a Marconi vertical antenna whether the plane of the radial conductors lies at or above the surface of the earth. The term "ground plane" embraces both the earth and the simulated earth.
If the vertical antenna is one-quarter wave length long, and the ground plane is a perfect conductor, the radiation resistance is half of that of the standard dipole. In practice, perfect conductivity in the ground plane is not achieved, and it is customary in antenna design to assume a ground plane resistance that reduces radiation resistance to 25 ohms which is one-half of what is assumed in dipole design.